Conventional lithographic techniques are well known in the manufacture of miniaturized electronic and magnetic circuits. In particular, photoresist layers are often used, where the photoresist layer is pattern-wise exposed and developed to provide a pattern which is then used for the deposition of a material or etching of the substrate on which the photoresist layer is located. The resolution obtainable in these photolithographic processes is limited by diffraction effects, which are in turn related to the wavelength of the light used in the exposure step.
In order to increase resolution, electron beam lithography is used. In this type of lithography, a deposited electron-sensitive resist layer is pattern-wise exposed, typically using an electron beam which is scanned across the resist layer and is turned on and off so as to form the desired exposure image in the resist layer. The resist is then developed in a manner similar to that used in photolithography. In electron beam lithography, resolution is limited primarily either by electron scattering effects in the material being radiated or by the diameter of the electron beam (if the beam diameter is too large). In particular, tightly focussed beams are provided by increasing the voltages that are used; however, increased energy beams lead to an increased energy of backscattered electrons, which have greater ranges and expose greater volumes of material. This in turn clouds the image produced by the electron beam exposure.
While it has been recognized in theory that electron scattering effects may be reduced by lowering the energy of the electrons in an electron beam, the minimum achievable beam diameter in conventional electron beam machines increases as the energy of the electrons in the beam is reduced. This occurs due to chromatic aberration in the magnetic and/or electronic lenses of such such apparatus, among other causes. Consequently, as the energy of the electrons in a conventional electron beam apparatus is reduced, the resolution actually deteriorates rather than improves because of the increasing beam diameter.
The need for high performance integrated devices and for further miniaturization has led to an improvement in providing such devices and circuits, as described in U.S. Pat. No. 4,785,189 by Oliver C. Wells, assigned to the present assignee. That reference recognizes that solutions to these two problems are not readily consistent since, if one of the resolution-limiting problems is corrected, the other is worsened. In order to overcome this, the reference utilized a different apparatus for providing a very narrow electron beam, the apparatus being a pointed tip from which electrons are emitted. Since the provision of a very narrow electron beam is achieved without large focussing voltages, the energies of electrons in the beam from the pointed tip are very small. This in turn solved the backscattering problem.
While a pointed tip or stylus is used in U.S. Pat. No. 4,785,189, such a technique and apparatus requires that the substrate contain a conductive layer in order to provide a return path for the tunneling current that is used to expose the electron-sensitive material. However, in the fabricaiton of many devices and circuits, substrates do not containn a conductive layer. Even if a conductive layer is present the insulating layer, which must be exposed, is often too thick to allow the passage of low energy electrons therethrough. Thus, while U.S. Pat. No. 4,785,189 is very useful for the exposure of very thin electron-sensitive resist layers or resist layers having a conductive layer located thereover, such a technique cannot be used where no highly conductive return path is provided for the tunneling electron current from the pointed tip, or stylus.
In a typical scanning tunnelling microscope (STM), a voltage of 0.1-1 volts is applied between the electron emitting tip and the conducting substrate, which is sufficient to drive a current in the nanoamp range continuously through the circuit. For a less highly conducting substrate return path, the currents are too weak to use to adjust the tip-to-substrate distance (Z) accurately. This is particularly apparent when the tip is to be scanned across the insulating substrate. In an STM, the pointed tip may be damaged if it has to be moved in a Z-direction to hunt for the substrate surface and then runs into the substrate surface. To solve this problem, the present invention uses a conductive pointed tip attached to a conducting cantilever to be able to accurately establish the desired tip-to-substrate distance even when the tip is scanned in the X-Y plane across the insulating substrate. This operation is similar to the movement of a pointed tip in an atomic force microscope (AFM) as described in U.S. Pat. No. 4,724,318.
Accordingly, it is a primary object of the present invention to provide an apparatus including a pointed tip and method for producing very narrow linewidths on substrates which do not include highly conductive layers serving as a current return path for electrons from the pointed tip.
It is another object of this invention to provide an apparatus including a pointed tip and method for affecting insulating materials, without the need for providing a highly conducting path through the insulating materials to the apparatus used to affect the insulating material.
It is another object of the present invention to provide an apparatus including a pointed tip, or stylus from which a very narrow beam of electrons can be emitted, and to utilize the narrow beam of electrons to affect an insulating material, there being no highly conductive return current path for said electrons.
It is another object of this invention to provide an apparatus and method for producing very-fine linewidth depositions on a highly insulating material or etched regions in the highly insulating material.
It is another object of the present invention to provide an improved technique which adapts the spatial resolution advantages possible with the use of the pointed tip or stylus to the provision of very narrow linewidth patterns on insulating materials of many thicknesses.